Oxide dispersion strengthened (ODS) alloys possess superior properties for high temperature applications; especially ODS alloys formed of superalloy materials. ODS alloys are distinguished from conventional alloys by the presence of dispersoids of fine particles and by an elongated grain shape which generally develops during a recrystallization heat treatment and/or hot and cold working. This particular grain structure enhances the high temperature deformation characteristics of ODS alloys by inhibiting the accumulation of inter-granular damage. As result of this and other properties, components fabricated from ODS alloys exhibit improved high-temperature creep strength and improved oxidation resistance as compared to conventional alloys.
The term “superalloy” is used herein as is understood in the art to describe a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures, as well as good surface stability. Superalloys typically include a base alloying element of nickel, cobalt or nickel-iron. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g., IN 700, IN 738, IN 792, IN 939), Rene alloys (e.g., Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C 263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g., CMSX-4, CMSX-8, CMSX-10) single crystal alloys.
ODS alloys, especially superalloys, are very difficult to weld and repair by conventional techniques (e.g., gas tungsten arc welding, laser welding, electron beam welding, etc.). Such fusion welding causes significant loss of strength. The alloys are furthermore difficult and uneconomical to process by less traditional processes such as friction welding.
ODS alloys are manufactured by mechanically alloying mixtures of powders. For example, metal powder such as alloys of iron aluminide, iron chromium, iron-chromium-aluminum, nickel chromium, or nickel aluminide, and oxides such as yttria (Y2O3) or alumina (Al2O3) are impacted in a ball mill. Shearing and smearing of the powders produces a fine mixture. A sealed container of the powder is then hot isostatically pressed and hot formed into a desired shape. High temperature heat treatment then provides stress relief and enlarges the grain size. Extraordinary strength is achievable with ODS materials. However, ODS processing is slow, expensive and provides limited control of part geometry. In addition, joining or repair of ODS parts is very difficult. Conventional arc and energy beam processes cause the fine oxide particles to segregate or coalesce, which degrades the result. Nickel based ODS superalloys are especially difficult to cold work and recrystallize.
Additional challenges associated with ODS alloys involve general shaping and joining of these materials. Shaping and joining techniques which preserve the microstructure and intrinsic strength of ODS alloys are severely limited, which often curtails their ability to be incorporated into high-temperature, load-bearing structures. For example, excessive heating of ODS alloys can cause the oxide to coalesce, leading to agglomeration such that the oxide dispersoids may no longer be effective in resisting slip at the grain boundaries. Melting of ODS alloys also results in “slagging off” of the oxide dispersoids reducing their strengthening ability. Since most ODS alloys derive their strength from an elongated grain structure, such disruption of the grain structure reduces strength.